Stainless steel vs. cupro-nickel exchangers

[Wizeman]

Hi all,

I'm wondering if anyone has had long-time experience with stainless steel exchangers on open-loop geothermal systems.

The reason I am asking is because the heat pumps with stainless steel exchangers are slightly less expensive (and more readily available where I am) than cupro-nickel exchangers.

So to re-iterate: is it OK to use a stainless steel exchanger in a water-source heat pump on an open ground loop?

Thanks,

-Vince

 

[vpl]

does the water have chlorides?



Patricia Lougheed

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[Wizeman]

I honestly don't know. I will have to ask.

 

[vpl]

Reason I'm asking is that presence of chlorides (which tend to be common in open loop water systems) cause SS to develop stress corrosion cracking -- which means replacement. A copper-nickel heat exchanger might be more expensive up front, but might last longer. You may need to do a cost comparison to decide which is your better option.

Patricia Lougheed

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[boo1]

The main factors in the preliminary selection of the heat exchanger (HE) materials are: physical properties, mechanical properties, corrosion resistance, fabrication and joining technology, experience, availability and cost. Thermal conductivity of the fin material has a direct effect on heat transfer efficiency, the higher the better.

Usually air-side heat transfer characteristics of fin-and-tube heat exchangers are determined from limited experimental data. But the overall heat transfer coefficient Uh is proportional to the tube and fin thermal conductivity. Changing the materials to a less conductive alloy will affect the system performance.

Stainless steel 316 (most commom HE) can exhibit stress corrosion cracking above about 60°C and is considered resistant to potable water with up to about 1000mg/L chlorides at ambient temperatures, reducing to about 500mg/L at 60°C.

 

[Wizeman]

Thanks boo1,

So basically if the well water doesn't go above 45 F (7.2 C) year-round and the chlorides stay below 1000mg/L I will be fine with Stainless steel?

Makes sense.

-Vincent

 

[boo1]

Analysis should shows you if the HE will perform within the required heat regection.

 

[MJCronin]

Do a chemical analysis of the liquids on both sides of the HX.

It's money well spent.

Copper nickle systems are good at chloride resistance, but you better not have any traces of ammonia in the system.

Be specific with your materials ......

Which particular cupro-nickle alloy are you considering ?

What is the temperature range of service ????

-MJC

 

[rmw]

VPL asked about chlorides; I'll ask about H2S. H2S is not uncommon around geothermal plants, in fact it is prevalent. Cu-Ni doesn't like H2S.

rmw